The Ising model is a statistical-mechanical model for explaining behavior of a magnetic substance. The Ising model is defined by spin that takes a binary of +1 and −1 (or 0 and 1 or upward and downward), an interaction coefficient showing interaction between spins and an external magnetic field coefficient for every spin. The Ising model enables calculating energy at that time on the basis of a provided spin array, interaction coefficients and an external magnetic field coefficient.
A search for a ground state of the Ising model is equivalent to an optimization problem for acquiring an array of spins that minimizes an energy function of the Ising model. It is known that the acquisition of a ground state of the Ising model which is a non-planar graph by topology is an NP-hard problem.
The search for a ground state of the Ising model is also equivalent, to a maximum cut problem known to be an NP-hard graph problem. Such a graph problem has broad applied skill to the detection of community in a social network, segmentation in image processing and others. Therefore, if a solver that searches for a ground state of the Ising model exists, the solver can be applied to such various problems.
Since there are a large number of the combinations of possible states of spins included in the Ising model, it is substantially impossible to search for all ground states in searching for a ground state. Therefore, to efficiently acquire a ground state, it is required to narrow down the combination of spins to be searched. However, since an evil effect also occurs that local optimum solution is selected by narrowing down and only an approximate ground state is acquired, a method of enabling acquiring better solution by avoiding local solution found in a process of a search is required.
For such a method, a method of searching for a ground state utilizing quantum fluctuation using a superconductive device is proposed. For example, for such a device, a device disclosed in Patent Literature 1 can be given.
Further, there is also a method of acquiring satisfactory solution using heuristics in which the deterioration of solution is temporarily allowed such as simulated annealing. When such a method is used, a ground state can be searched at high speed by using hardware disclosed in Nonpatent Literature 1 for example.